The present invention relates to surface wave devices. More particularly, the present invention relates to an improved electro-acoustic transducer structure for acoustic surface wave devices.
Much work has been done recently to improve the performance of surface wave devices. Basically a surface wave device consists of a smooth substrate of a piezoelectric type material capable of propagating acoustic surface waves. Electro-acoustic transducers are attached to or held in intimate contact with the substrate in order to convert electrical rf energy to surface waves on the substrate. When an electro-magnetic field is impressed on the piezoelectic material, a physical deformation of the material results. If the electro-magnetic field is made time-variant the resulting deformation will also be time-variant. The deformation effects will propagate through the material as acoustic or sound waves, much like sonar waves, through water. A second electro-acoustic transducer held in intimate contact with the substrate will sense the deformation effects and convert the surface wave into an electrical rf energy.
One of the significant advantages of surface wave devices is that surface acoustic waves travel considerably slower on the substrate than do electro-magnetic waves in free space. This causes the wavelengths to be far shorter and devices such as filters, delay lines, and couplers can be miniaturized using surface wave technology. In the past, one significant problem which was encountered in the design of surface wave devices, such as low shaped factor bandpass filters with accurately control frequency response characteristics, was synchronous reflection. During the operation of surface wave device transducers, the electrodes provide effective acoustic wave impedance discontinuities along the substrate surface causing reflection of surface waves. These reflections will add in-phase, in prior art transducers having electrode widths and spacings of one quarter wavelength along the direction of propagation, thus creating a large amplitude reflective wave which results in frequency response distortion in the bandpass filter or in a delay line employing these transducers.
One prior art solution to this reflection problem consisted of using a double finger electrode with one-eighth wavelength width interdigital fingers used in pairs. The use of one-eighth wavelength width fingers, however, significantly limits the upper frequency at which a surface wave device can be fabricated due to practical limitations encountered in making transducer fingers of sufficiently small dimensions.
Another solution that has been proposed in the past consisted of using the double finger electrode geometry in a transducer operating at the third harmonic of its fundamental spacing as taught in U.S. Pat. 3,803,520 which issued to Thomas Bristol, et al. This technique allows a fabrication of a surface wave device at three times the frequency which would be possible with a double finger transducer electrode operated at its fundamental frequency. However, this technique results in a loss of tap weight accuracy and reduced efficiency in the generation of surface waves on the substrate which tends to result in the inability to control the frequency response, shape factor, and low level sidelobes particularly those greater than 35 db below the peak response of the surface wave device being fabricated.
Furthermore, the fabrication of low shape factor filters with low level sidelobes requires the use of one or more accurate tap weighted transducers. Tap weighted transducers are those in which both the phase and amplitude of energy launched by each electrode onto the substrate surface is accurately controlled in order to shape the impulse response of the filter or delay line being fabricated. This in turn shapes the frequency response of the given device which is mathematically related to the impulse response by the Fourier Transform.
In the design of bandpass filters requiring low shape factors and low sidelobe levels it is sometimes necessary to incorporate two weighted tap transducers, one to act as the input transducer and the other as the output transducer since sidelobe levels below approximately 30 db cannot be readily achieved using only one weighted transducer. At least one of these transducers must be capable of integrating the surface wave energy across the width of the beam. Thus a series weighted transducer is generally used for at least one of the required transducers and either a series weighted, modified series weighted, or overlap weighted transducer may be used for the other transducer as taught in "Design Problems in Surface Wave Filters" by Ronnekleiv, Skeie and Hanebrekke; IEE International Specialists Seminar on Component Performance and Systems Applications of Surface Acoustic Wave Devices, Aviemore, Scotland, Pages 141--151, 1973.
In order to achieve the highest possible operating frequency while controlling the frequency response and sidelobe levels of the surface wave devices accurately, an electrode structure is required which can be used in all types of amplitude weighted transducers, which gives reflection suppression and has relatively large finger widths compared to an acoustic wavelength.
In view of the foregoing, it should now be understood that it would be desirable to provide a surface wave device having an improved electro-acoustic transducer.
Accordingly, one of the objects of the present invention is to provide a surface wave device having a reflectionless transducer which does not employ the double finger configuration.
Another object of the invention is to provide a surface wave device having an electro-acoustic transducer electrode which can be implemented in all types of amplitude weighted transducers and more specifically overlap and series weighted transducers and variations thereof.
Yet another object of this invention is to provide a reflectionless transducer electrode having a reduced fabrication resolution equipment.
Still another object of the invention is to provide a surface wave transducer capable of operating at higher frequencies than heretofore possible while maintaining high efficiency conversion of rf to surface wave energy and accurate control of the frequency and impulse responses of a surface wave device in which the transducer is being used.